Texas leaving the union...
Could we sustain ourselves? And, would we have to?
Texas has an abundance of natural resources, largely due to the fact that it's such a large landmass. There is a tremendous fossil fuel reserve in the Barnett shale, and combined with all other extraction methods currently in play in Texas, it could sustain itself on its own fossil fuel energy alone for ~30 years on proven reserves, and potentially 150+ years on "technically recoverable" resources.
Since its highly unlikely that an isolationist position would be employed, this isn't much a concern... but it's nice to have the option. Without federal regulations, extraction methods and activity would greatly increase, and new reserves would likely be identified leading to an absolute glut of fossil fuel energy in Texas to use for trade.
The climate is fairly temperate most of the year, but our soil isn't always the best. It's not easy to grow non-native crops here without serious amendments to the soil. Native crops, however, grow all too well.
We have abundant cotton, corn, wheat, feed grains, oats, and lots of pasture land for our large cattle industry. We also have droughts... which make things very difficult from time to time.
We have the largest branch of National Guardsmen by state, at ~23,000. We have a long history of experience securing our borders, and would likely become exceptionally efficient at it if no longer ham-hocked by the federal government. I can say for sure, the southern wall would get finished. It would become a matter of actual "national security" to stop the flow of illegal aliens and illegal drugs into Texas. My apologies to the other states which will absolutely suffer an influx as a result.
Along with our National Guard, we also have a well armed society.
I used ChatGPT to give me an estimate of firearms in Texas, and how many that would equate to per household. It stated that a 2018 (so pre-COVID) survey of civilian owned firearms in Texas revealed 21.5 million. With 29 million people in the state, averaged into homes of 2.84 people, that would be 556,552 firearms per household.

I know that's comically misleading, but I found it very funny. The point is, without federal regulations, that number would go up and it's already the 2nd largest civilian firearm count in the nation (behind California, of all places, at 25.1m).
I know this oversimplifies a lot of things, but it covers some big basics... Energy, Food, Security.
In a follow-up, I'll go further into whether it's actually a good idea or not.